With the advent of information-oriented society, an optical disc with a large capacity for recording a great amount of information has been widely used. A great variety of optical discs are developed nowadays. For example, CD-ROM (Compact Disc Read Only Memory) is a kind of optical disc that has recorded data therein for read purpose only. CD-R (Compact Disc-Recordable) and CD-RW (Compact Disc-ReWritable) are two other kinds of optical discs, different from CD-ROM, allowing data to be written to the optical disc by a user.
CD-R exhibits the writable feature due to a special dye layer coated thereon. When the optical head emits programmed laser light onto the dye layer of the CD-R, specified portions of the dye layer receiving the light are heated up to 250 degrees C. and thus melted to form “pits”, otherwise remain as “lands”. Data are then recorded in the optical disc by way of the resulting pits and lands. FIG. 1(a) depicts an optical disc 10 with exemplified pits 11 and lands 12 in a partially enlarged view. An optical head 2 is disposed at a side of the optical disc 10, records data into the optical disc 10 according to a write strategy, and realizes information from the optical disc 10 according to the previously formed pit-land configuration of the optical disc 10.
CD-RW, compared to CD-R, is advantageous because of its erasable and rewritable feature. The data recording in a CD-RW disc is dominated by phase change, e.g. the differential reflection rates between the crystalline phase and the amorphous phase of the CD-RW material.
A CD-RW disc includes a recording material layer formed of an alloy of four metals, i.e. silver, indium, antimony and tellurium, and capable of switching between the crystalline phase and amorphous phase. The phase change of the recording material layer is rendered by diode laser power. The power typically applied for writing is from 8 mW to 14 mW and 4˜8 mW for erasing, and the bias power is 0˜1 mW. The melting point of the recording material layer is about 600 degrees C., while the crystallization temperature is about 200 degrees C. Likewise, lands and pits are formed in a CD-RW disc to record data. As shown in FIG. 1(b), for forming a land structure 22 in a specified region of a CD-RW disc 20, an erasing procedure is performed. Meanwhile, the laser power pulse signal 200 is emitted by the optical head 2 and maintained at a level 201 for a certain period of time to have the temperature of the erased region remain between the melting point and the crystallization temperature so as to crystallize. On the other hand, for forming a pit structure 21 in a specified region of the CD-RW disc 20, a writing procedure is performed. In the writing procedure, the laser power pulse signal 200 emitted by the optical head 2 is switched between a level 202 higher than the level 201 for erasing and a level 203 lower than the level 202. In this way, the recording material layer in the specified region will reach the melting point in a short time. Then, the laser power pulse signal 200 is suspended so as to cool the recording material layer promptly. The formation of a relatively long pit would require repetitive pulse operations between the levels 201 and 203.
When the optical disc is inserted into an optical recording/reproducing apparatus to be read, a laser beam is emitted from an optical head of the optical recording/reproducing apparatus and reflected by these pits and lands with different reflection rates. For example, while the majority of the light emitted onto the lands is reflected, only 10 to 20 percents of the light emitted on to the pits can be reflected due to destructive interference. The reflected light is then received and processed by the optical head to reveal the previously recorded information.
As writing quality varies with types or makes of optical discs if the same writing condition is applied, different write strategies are given by the manufacturers for writing a variety of optical discs so as to improve writing quality. A write strategy is generally necessary to take a plurality of parameters into consideration, e.g. power for writing, power for erasing and bias power. Therefore, to build up a write strategy for each optical disc is a huge, complicated and time-consuming task.